Why is the First Ionization Enthalpy of Magnesium (Mg) Higher than Aluminum (Al)?

The ionization enthalpy of magnesium (Mg) is higher than that of aluminum (Al), primarily due to differences in their atomic structures, effective nuclear charges, and electron shielding effects. Understanding these factors can provide insight into why Mg requires more energy than Al to initiate the removal of its valence electron.

Atomic Structure and Electron Configuration

Magnesium has the electron configuration of [Ne] 3s2, while aluminum has [Ne] 3s2 3p1. This means that aluminum has one more electron in the 3p orbital compared to magnesium. This additional electron in aluminum faces significant shielding from inner electrons and the 3s electrons, which influences the ionization energy.

Effective Nuclear Charge

As you move from magnesium to aluminum, the number of protons in the nucleus increases, thereby increasing the nuclear charge. However, the additional electron in aluminum is added to a higher energy level (3p) and is also shielded by inner electrons and the 3s electrons. This shielding effect reduces the effective nuclear charge felt by the outermost electrons, making it easier for aluminum to donate an electron with less energy compared to magnesium.

Electron Shielding

The 3p electron in aluminum experiences more shielding from the inner electrons compared to the 3s electrons in magnesium. This increased shielding effect makes the outermost electron in aluminum easier to remove, leading to a lower ionization enthalpy for Al compared to Mg.

Energy Levels

The 3p orbital is higher in energy and further from the nucleus compared to the 3s orbital. Because the 3p orbitals are less tightly bound to the nucleus, they require less energy to remove an electron. This is a key reason why aluminum has a lower ionization enthalpy than magnesium.

Stability and Ionization Energy

Another factor to consider is the electronic configuration and stability of the elements. Magnesium has a completely filled 3s2 subshell, which adds to its overall stability. To remove an electron from a fully filled 3s subshell requires more energy than removing an electron from a partially filled 3p subshell in aluminum. This additional stability in the Mg atom further contributes to its higher ionization enthalpy.

Conclusion

In summary, the combination of increased electron shielding and the higher energy level of the 3p electron in aluminum leads to a lower first ionization enthalpy for Al compared to Mg. Additionally, the stability of the fully filled 3s subshell in Mg also plays a critical role.